You can go the simple iron filings on a white piece of paper route if you want to see the invisible fields surrounding a magnet. But I prefer this DIY contraption which carves 3D foam sculptures of them instead.

You don't need to be a physicist, a PhD student, a geek, or even a member of the Insane Clown…
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Using a simple electric motor with a drill bit attached that works like a CNC machine, the fairly rudimentary carver is guided by an analog magnetic sensor detecting the strong field around a rare Earth magnet. As the rig moves back and forth making its cuts, a simple microcontroller ensures that whenever a magnetic field is detected, the drill bit is pulled back. So over time you're left with a 3D sculpture that roughly represents the fields surrounding the magnet. And up next, a larger version that uses a halitosis detector so you can see the similar 3D field around your head that all your friends are avoiding. [Make via BoingBoing]